<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>For循环优化</title>
</head>
<body>
    <p class="btn">add</p>
    <p class="btn">add</p>
    <p class="btn">add</p>
    <p class="btn">add</p>
    <p class="btn">add</p>
    <p class="btn">add</p>
    <p class="btn">add</p>
    <p class="btn">add</p>
    <p class="btn">add</p>
    <p class="btn">add</p>

    <script>
        var aBtns = document.getElementsByClassName('btn')

        for(var i=0;i<aBtns.length;i++) {
            console.log(i)
        }
        
        for(var i=0,len= aBtns.length;i<len;i++) {
            console.log(i)
        }
    </script>
</body>
</html>